1. Field of the Invention
This invention broadly relates to the use of digital data in the practice of dental methods. More particularly, the present invention relates to methods of preparing virtual dentition models and methods of fabricating dental appliances such as retainers from virtual models.
2. Description of the Related Art
A wide variety of dental appliances are known in the art. Some types of dental appliances are used in orthodontic treatment to move crowded, crooked or misaligned teeth to correct positions in the oral cavity. Other types of dental appliances include retainers, which are often used to maintain the position of the patient's teeth following orthodontic treatment.
Orthodontic treatment is often carried out by the use of tiny slotted appliances known as brackets that are secured to the patient's teeth by an adhesive. A resilient archwire is placed in the slot of each bracket and forms a track to guide movement of the teeth to desired locations. Ends of the archwire are often connected to small appliances known as buccal tubes that are secured to the patient's molar teeth. In some instances, brackets and buccal tubes are not directly bonded to the surfaces of the patient's teeth but are instead welded or brazed to small metallic bands that, in turn, are placed over the corresponding teeth in encircling relation.
Often, a set of brackets, buccal tubes and archwire is provided for each of the patient's upper and lower dental arches. The brackets, buccal tubes and archwires are commonly referred to collectively as “braces”. In many instances, braces are worn by patients for a period of time extending from one to two years.
Once the patient's teeth have reached their final desired positions as a result of the corrective forces provided by the braces, the braces (including the brackets, archwires, buccal tubes and other components) are removed from the patient's teeth. However, at this point in time the patient's teeth are often relatively mobile and may tend to move toward previous positions in the dental arches if not held in place. For this reason, dental retainers are often used at the conclusion of orthodontic treatment to immobilize the teeth until such time as sufficient bone structure has grown along roots of the teeth to retain the teeth in desired positions.
Dental retainers are commonly fabricated by commercial dental laboratories using a plaster of Paris or “stone” model of the patient's teeth that resembles the appearance of the teeth in their final or finish positions. Often, retainers are made using thermoplastics such as acrylics, ethylene/propylene copolymers, polyvinylchloride, polycarbonate and polyethylene terepthalate. Thermosetting acrylics are also used. Dental retainers often also incorporate metallic members such as wires, springs and frameworks for engagement with the teeth. Some types of dental retainers are removable by the patient when desired such as during mealtimes, while other types of dental retainers are fixed in place by, for example, an adhesive.
Certain types of dental retainers are fabricated by placing a custom-bent wire on the stone model. Next, a quantity of an acrylic material is arranged along portions of the wire to form the body of the retainer for placement next to the patient's palate. After the acrylic has hardened, the retainer is removed from the stone model and trimmed as desired. The retainer is then polished and shipped to the orthodontic practitioner.
It is often deemed best for an orthodontic patient to wear a retainer as soon as the braces have been removed from the patient's mouth. For this reason, it is desirable to begin fabrication of the customized retainer while the patient is still wearing braces so that the retainer is available at the practitioner's office immediately after the braces have been removed from the patient's mouth. To this end, an impression of the patient's teeth while wearing braces is often taken approximately one or two months before the estimated completion of orthodontic treatment so that a stone model of the patient's dentition can be prepared and used to fabricate the custom retainer.
However, the practice of taking an impression of the patient's teeth while wearing braces often presents difficulties. For example, the impression material often extends into recesses, cavities and other structure of the brackets, buccal tubes and archwires and hardens. As the impression material is removed from the patient's oral structure, the impression material may tear or fracture in areas adjacent the orthodontic appliances, which may cause areas of distortion of the resulting stone model. In some instances, removal of the impression material may also bend or distort the orthodontic components or cause the components to detach from the teeth.
In addition, when an impression is taken while the patient is still wearing braces, the resulting stone model will include portions that represent the teeth as well as portions that represent the braces. However, the portions of the stone model that represent the braces must often be removed in order to provide a stone model suitable for fabrication of a retainer. Removal of the stone braces is usually carried out by a laboratory technician in a tedious manual process using, for example, a hand-held rotary tool with a burr or grinding head. Unfortunately, this manual process tends to increase both the cost and the fabrication time of the resulting retainer. Moreover, this process calls for the technician to estimate the missing surface contour of each tooth while grinding off the stone braces, and this estimation is somewhat subjective.